Life History of the Plain Nawab

Life History of the Plain Nawab (Polyura hebe plautus)

Butterfly Biodata:
Genus: Polyura Billberg, 1820
Species: hebe Butler, 1866
Subspecies: plautus Fruhstorfer, 1898
Wingspan of Adult Butterfly: 65mm
Local Caterpillar Host Plants: Adenanthera pavonina (Red Saga), Falcataria moluccana (Albizia), Parkia speciosa (Petai), all belong to the family Leguminosae, sub-family Mimosoideae.

Physical Description of Adult Butterfly:
The forewing has a strongly arched costa, a pointed apex and a concave termen. The hindwing has a pair of short stubby tails. Above, the Plain Nawab is greenish white and the forewing has a broad black border decreasing markedly from the apex towards the tornus and base of the costa, and a moderately large greenish white spot in space 5. The hindwing has a series of marginal spots. The Singapore subspecies of P. hebe, represented by the race plautus, is different from its Malaysian cousin P. hebe chersonesus in having a broad black border on the hindwing. Beneath, the Plain Nawab has a large, pale silvery-green median patch covering about a quarter of the wing. The broad wing borders are mainly brown to dark brown in color.


Another Plain Nawab found perching on a leaf in a hill park


A Plain Nawab perching on a leaf with open wings.

Field Observations of Butterfly Behaviour: The adults are medium-sized, heavy-bodies butterflies with rapid, strong and sometimes erratic flights. It has a habit of perching high on a spot in a tree, and keeps returning to the same perch after making swift flights in the vicinity to chase intruders away. The adults have also been observed to puddle on wet ground, carrion, faeces and tree sap. Plain Nawab is relatively common and widely distributed in Singapore, with frequent sightings made in both the nature reserves and Southern Ridges.


A Plain Nawab feeding on tree sap


A Plain Nawab puddling on wet ground in the nature reserves

Early Stages:
The early stages of the Plain Nawab have been locally observed to feed on leaves of three plants in the Leguminosae family. Of the three, the Red Saga is most popularly utilized with regular sightings of caterpillars on young saplings in Southern Ridges where the plant is commonly planted. Red Saga can grow to a rather large tree. Its leaves are bipinnate, with 3 to 6 pairs of side stalks, and 9 to 15 leaflets on each side stalk. The seed pods are curved and scarlet seeds.


Host plant : Adenanthera pavonina

A mating pair of the Plain Nawab

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A mother Plain Nawab laying an egg on Red Saga in a hill park.


Left: fresh egg; Right: mature egg with a glimpse of the head capsule

The egg takes 4 days to hatch, and the newly hatched has a body length of about 3-3.5mm. The young caterpillar eats the entire egg shell as its first meal. Its body is initially creamy yellow but turning green a day or two later. It has a black head with 2 pairs of horns, the lower of which being shorter and straight, and the upper one being longer and curved. There is also pair of biege anal processes. Between feeds, the young caterpillar rests along the midrib near the leaf tip on the upperside. From this "base camp", the caterpillar ventures out to feed on nearby leaflets. Larger caterpillars in later instars spin a silk mat across several leaflets in order to provide a sufficiently large "base" on which to rest.


Two views of a newly hatched Plain Nawab caterpillar, length: 3.5mm


Left: Two Plain Nawab caterpillars (in early 1st instar) sighted resting on the upper surface
of Red Saga leaves in the Southern Ridges.
Right: A close-up view of one caterpillar in the left panel.

After 4-5 days of feeding, the 1st instar caterpillar grows to a length of about 6.5-7mm. The caterpillar stays dormant in its base camp with the new head capsule growing progressively larger behind the current one. The caterpillar moults to the next instar about half a day later.
1st instar caterpillar, late in this stage, length: 6mm.

Besides the increase in size, in particular the head capsule, the 2nd instar caterpillar has similar appearance as the 1st instar caterpillar. After five days in this instar, and having the body length increased to about 12mm, the caterpillar moults again.


Newly moulted 2nd instar caterpillar with old head capsule nearby, length: 6.5mm



A 2nd instar Plain Nawab caterpillar resting in its "base camp" on the upper surface
of a Red Saga leaf in the Southern Ridges.

The head capsule of the 3rd instar caterpillar is pale yellowish brown with wide vertical streaks in dark brown. The pair of anal processes has become shorter in length and prominence. Light yellowish stripes fringes the body just below the spiracles on the abdominal segments. This instar takes 4 to 5 days to complete with body grown to about 16-17mm in length.


Three views of a 3rd instar caterpillar, length: 15mm

The head capsule of the 4th instar caterpillar is yellowish green with two pairs of stripes in darker green. This instar lasts a further 8-9 days with body length reaching about 25-26mm. In the last few days of the instar, pale green crescent marks start to appear on each body segment, giving us a hint of what the next instar will bring.


Upper left: late 3rd instar; Lower right: newly moulted 4th instar;
Right: head capsule of the 4th instar caterpillar


Two views of a 4th instar caterpillar, length: 23mm


A late 4th instar Plain Nawab caterpillar resting on a "base camp" built from joining several
adjacent leaves of the Red Saga. Found in the Southern Ridges.

The 5th instar caterpillar closely resembles the 4th instar. The most noticeable change is the silvering green crescent marks which stand in stark contrast to the dark green base color. The pair of anal processes is so small that the anal segment takes on a near rectangular outline.


5th instar caterpillar, newly moulted, length: 22mm


5th instar caterpillar, late in this stage, length: 48mm


Head capsule of the 5th instar caterpillar

The 5th instar lasts for 10-13 days, and the body length reaches up to 48-50mm. Toward the end of this instar, the body gradually shortens in length. The fully grown caterpillar soon abandons its "base camp" and goes in hunt for a pupation site. Eventually the caterpillar comes to rest on a spot on the under surface of a stem. There it spins a silk pad to which it attaches its claspers (anal prolegs). The pre-pupatory larva then hangs vertically, typically with its body curled up.

A pre-pupa of the Plain Nawab in its curled up posture

Pupation takes place a day later. The pupa suspends itself from the same silk pad but now with its cremaster. The pupa has a berry like appearance with its thick and cylindrically oval shape. It is green but streaked with abundant white and features a broad head. Length: 17-19mm.

A time-lapse sequence of the pupation event for one Plain Nawab caterpillar


Three views of a berry like pupa of the Plain Nawab.

The pupal period lasts for 9 days, and the pupa turns dark to reddish brown the night before eclosion. The large pale green patches on the forewings also become visible through the pupal skin at this stage.


Three views of a mature pupa of the Plain Nawab.

Eclosion takes place the next morning. The pupal case first cracks open with the adult butterfly making its way out. It quickly turns around and perches on the underside of the pupal case to "dry" and expand its wings. A few hours later, the adult butterfly makes the first flight of its life.

A time-lapse sequence of the eclosion event for one Plain Nawab caterpillar


A newly eclosed Plain Nawab

References:

• The Butterflies of The Malay Peninsula, A.S. Corbet and H.M. Pendlebury, 4th Edition, Malayan Nature Society.
• The Butterflies of Hong Kong, M. Bascombe, G. Johnston, F. Bascombe, Princeton University Press 1999

Text by Horace Tan, Photos by Benedict Tay, Khew SK and Horace Tan